[RFC][PATCH 0/8] PM: Implement autosleep and "wake locks"

[Rafael J. Wysocki]

Hi all,

This series tests the theory that the easiest way to sell a once rejected
feature is to advertise it under a different name.

Well, there actually are two different features, although they are closely
related to each other.  First, patch [6/8] introduces a feature that allows
the kernel to trigger system suspend (or more generally a transition into
a sleep state) whenever there are no active wakeup sources (no, they aren't
called wakelocks).  It is called "autosleep" here, but it was called a few
different names in the past ("opportunistic suspend" was probably the most
popular one).  Second, patch [8/8] introduces "wake locks" that are,
essentially, wakeup sources which may be created and manipulated by user
space.  Using them user space may control the autosleep feature introduced
earlier.

This also is a kind of a proof of concept for the people who wanted me to
show a kernel-based implementation of automatic suspend, so there you go.
Please note, however, that it is done so that the user space "wake locks"
interface is compatible with Android in support of its user space.  I don't
really like this interface, but since the Android's user space seems to rely
on it, I'm fine with using it as is.  YMMV.

Let me say a few words about every patch in the series individually.

[1/8] - This really is a bug fix, so it's v3.4 material.  Nobody has stepped
  on this bug so far, but it should be fixed anyway.

[2/8] - This is a freezer cleanup, worth doing anyway IMO, so v3.4 material too.

[3/8] - This is something we can do no problem, although completely optional
  without the autosleep feature.  Rather necessary with it, though.

[4/8] - This kind of reintroduces my original idea of using a wait queue for
  waiting until there are no wakeup events in progress.  Alan convinced me that
  it would be better to poll the counter to prevent wakeup_source_deactivate()
  from having to call wake_up_all() occasionally (that may be costly in fast
  paths), but then quite some people told me that the wait queue migh be
  better.  I think that the polling will make much less sense with autosleep
  and user space "wake locks".  Anyway, [4/8] is something we can do without
  those things too.

The patches above were given Sign-off-by tags, because I think they make some
sense regardless of the features introcuded by the remaining patches that in
turn are total RFC.

[5/8] - This changes wakeup source statistics so that they are more similar to
  the statistics collected for wakelocks on Android.  The file those statistics
  may be read from is still located in debugfs, though (I don't think it
  belongs to proc and its name is different from the analogous Android's file
  name anyway).  It could be done without autosleep, but then it would be a bit
  pointless.  BTW, this changes interfaces that _in_ _theory_ may be used by
  someone, but I'm not aware of anyone using them.  If you are one, I'll be
  pleased to learn about that, so please tell me who you are. :-)

[6/8] - Autosleep implementation.  I think the changelog explains the idea
  quite well and the code is really nothing special.  It doesn't really add
  anything new to the kernel in terms of infrastructure etc., it just uses
  the existing stuff to implement an alternative method of triggering system
  sleep transitions.  Note, though, that the interface here is different
  from the Android's one, because Android actually modifies /sys/power/state
  to trigger something called "early suspend" (that is never going to be
  implemented in the "stock" kernel as long as I have any influence on it) and
  we simply can't do that in the mainline.

[7/8] - This adds a wakeup source statistics that only makes sense with
  autosleep and (I believe) is analogous to the Android's prevent_suspend_time
  statistics.  Nothing really special, but I didn't want
  wakeup_source_activate/deactivate() to take a common lock to avoid
  congestion.

[8/8] - This adds a user space interface to create, activate and deactivate
  wakeup sources.  Since the files it consists of are called wake_lock and
  wake_unlock, to follow Android, the objects the wakeup sources are wrapped
  into are called "wakelocks" (for added confusion).  Since the interface
  doesn't provide any means to destroy those "wakelocks", I added a garbage
  collection mechanism to get rid of the unused ones, if any.  I also tought
  it might be a good idea to put a limit on the number of those things that
  user space can operate simultaneously, so I did that too.

All in all, it's not as much code as I thought it would be and it seems to be
relatively simple (which rises the question why the Android people didn't
even _try_ to do something like this instead of slapping the "real" wakelocks
onto the kernel FWIW).  IMHO it doesn't add anything really new to the kernel,
except for the user space interfaces that should be maintainable.  At least I
think I should be able to maintain them. :-)

All of the above has been tested very briefly on my test-bed Mackerel board
and it quite obviously requires more thorough testing, but first I need to know
if it makes sense to spend any more time on it.

IOW, I need to know your opinions!

Thanks,
Rafael

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